1. Field of the Invention
This invention relates to a magnetoinductive flowmeter for measuring the flow rate of a moving medium, with a measuring tube and with an insert provided in said measuring tube for conducting the medium, said insert including at least one first part and one second part of which the first part is introduced into the measuring tube from one end of the measuring tube and the second part, introduced from the other end of the measuring tube, connects to the first part in leak-proof fashion. A magnetoinductive flowmeter of that type has been described earlier in WO 00/47954 A1.
Magnetoinductive flowmeters have been well known for some time and are widely used in various fields of application. The fundamental concept of a magnetoinductive flowmeter goes back to Faraday who in 1832 proposed employing the principle of electromagnetic induction for flow-rate measurements.
According to Faraday's law of induction, a flowing medium containing charge carriers and passing through a magnetic field will generate an electric field intensity perpendicular to the direction of flow and to the magnetic field. In a magnetoinductive flowmeter, Faraday's law of induction is employed in that a field coil serves to generate a magnetic field with a magnetic-field component that extends in a direction perpendicular to the direction of flow. Within this magnetic field, each volume element of the medium flowing through the magnetic field and containing a certain number of charge carriers contributes the field intensity generated in the latter to a measuring voltage that can be collected via measuring electrodes.
2. Background Information
In conventional magnetoinductive flowmeters, these measuring electrodes are designed for contact with the flowing medium either in electroconductive or in capacitive fashion. A salient feature of magnetoinductive flowmeters is the proportionality between the measured voltage and the flow velocity of the medium, averaged across the diameter of the measuring tube, i.e. between the measured voltage and the volume of flux.
It is at least in the area of the measuring electrodes that the interior of the measuring tube must be electrically nonconducting or be provided with an electrically insulating liner. This can be accomplished by means of an insert as referred to above. A magneto-inductive flowmeter featuring this type of insert has been described, for instance, in DE 100 46 195 C2.
According to that document, at least one end section of the insert is elastic while the center section is rigid. The center section consists of a first material and the end section is made from a second material, which second material has a lower modulus of elasticity than does the first material. The insert composed of these materials is prefabricated as a one-piece module that can be introduced into the measuring tube of the magnetoinductive flowmeter by deforming the elastic end section enough to permit being passed through the interior of the measuring tube. After the insert is introduced into the measuring tube and its elastic end section has been passed through the other end of the measuring tube, the elastic section can regain its relaxed state. In this fashion, an insert can be produced with a flange at each end that extends outside the measuring tube and whose outer diameter is larger than the inside diameter of the measuring tube. While introducing an insert of that type in the measuring tube is relatively manageable, it would appear that producing the insert from at least two different materials with mutually different moduli of elasticity poses a problem.